1. Field of the Invention
The present invention relates to an objective lens driving apparatus of an apparatus for optically recording or reproducing information on a recording medium.
2. Description of the Prior Art
A conventional objective lens driving apparatus comprises a movable part consisting of an objective lens of an apparatus for writing or reading optical information on a disk-shaped recording medium installed at a specific position corresponding to a spindle motor by means of a loading mechanism and a lens holder mounting said objective lens, a base, four bar-shaped elastic support members having one end fixed on said base and supporting said movable part movably in the optical axis direction vertical with respect to the disk-shaped recording medium and in the radial direction parallel to the disc-shaped recording medium, an optical axis direction driving coil having a rotating axis in the optical axis direction and fixed on the movable part, a radial direction driving coil having a rotating axis in the radial direction and fixed on the movable part, and a driving magnet fixed on the base being disposed opposite to the side face of the optical axis direction driving coil and radial direction driving coil.
The operation of thus composed objective lens driving apparatus is generally as described below.
In the first place, before the optical axis direction driving coil and radial direction driving coil are energized, that is, in the initial state when the objective lens is not driven, the disk-shaped recording medium performs a loading action by the loading mechanism. That is, the disk-shaped recording medium is lowered in the direction of approaching the objective lens in the optical axis direction by the loading mechanism, and is installed at a specific position confronting the spindle motor.
Upon completion of the loading action of the disk-shaped recording medium, the focus control of the objective lens on the recording plane of the disk-shaped recording medium is started, and the objective lens is driven in the optical axis direction.
Driving of the objective lens in the direction of the optical axis is achieved as the optical axis direction driving coil fixed on the lens holder mounting the objective lens receives an electromagnetic force obtained as it orthogonally crosses the magnetic flux generated by the driving magnet installed on the base, and moves parallel since it is supported by the four bar-shaped elastic support members having one end fixed on the base.
In the objective lens driving apparatus, there is a growing demand for reduction of size and thickness of the apparatus, and accordingly a lens of small size is often used for the objective lens. Therefore, when using a small objective lens without lowering the precision of recording and reproducing, it is necessary to use an objective lens having a short working distance.
In the above construction, however, when the disk-shaped recording medium is lowered in the direction of approaching the objective lens in the optical axis direction by the loading mechanism and installed at a specified position with respect to the spindle motor, the disk-shaped recording medium may not be always lowered correctly depending on the mechanical precision of the loading mechanism or other circumstances. Therefore, when using an objective lens with a short working distance, the shorter the working distance of the objective lens, the closer approaches the objective lens to the installing position of the disk-shaped recording medium, which gives rise to an increased risk of collision of the disk-shaped recording medium against the objective lens.